I bet JohnK or 2x6 can help with this better than I.
However, I've read many times of people building and mouting sub enclosures in the floor or walls of their houses. These typically are DIY projects; Peter rightly points out that the engineering of a Hsu or SVS intends the product to "live" in the same room as the listener.
But how about working on a design that lives in the crawlspace rather than the attic space? I think you'd be much happier, unless you rearrange your seating so that you are sitting on the ceiling instead. If you have a DIY system that is "ported" into the room (via a floor vent - like a heat register almost) and structurally coupled to the floor joists, you do have some advantages. Namely, that the size of the enclosure ceases to be a design issue. Of course, it won't be "cheaper" and may not be "better" than an off-the-shelf design. But there are options.
